In a cellular network, each cell comprises a primary radio station, like a base station, or a Node B, or an eNB communicating with a plurality of secondary stations, like mobile stations, or User Equipments. To be able to send data to the primary station on some uplink channels, a secondary station must typically have an allocated resource (e.g. time slot, frequency subcarrier, and/or code).
Many communication systems operate using a centralised scheduler which is responsible for allocating transmission resources to different nodes. A typical example is the uplink of the UMTS LTE (Long Term Evolution), where the uplink transmissions from different secondary stations are scheduled in time and frequency by the primary station. The primary station transmits a “scheduling grant” message to a secondary station, indicating a particular time-frequency resource for the secondary station's transmission, typically around 3 ms after the transmission of the grant message. The grant message also typically specifies transmission parameters as the data rate and/or power to be used for the secondary station transmission.
In order for the primary station to issue appropriate grants, it needs to have sufficient information about the amount and type of data awaiting transmission in the buffer of each secondary station.
In LTE, several types of buffer status report (BSR) messages are therefore defined, which may be transmitted from a secondary station to the primary station when certain triggers occur. The current version of 3GPP TS36.321 is incorporated by reference.
The Buffer Status reporting procedure is used to provide the serving primary station with information about the amount of data in the uplink buffers of the secondary station. Two kinds of Buffer Status Reports are used depending on the events. A short Buffer Status Report (BSR) comprises the identity of a single group of logical channels, together with a 6-bit indicator of the amount of data corresponding to that group of logical channels currently residing in the secondary station buffer awaiting transmission. A long BSR comprises four concatenated short BSRs, each corresponding to a different group of logical channels.
A problem with the BSR procedure defined above is that a secondary station is only permitted to transmit a BSR if it has a granted resource in which to transmit. If new data arrives in a secondary station's buffer and the secondary station has no granted resource in which to transmit the data or to send a BSR to indicate that it has data awaiting transmission, the secondary station must either wait until a grant is received, or transmit a simpler version of the BSR which can be transmitted using some specially-designated resources which can be used without a specific granted resource. This simpler version of the BSR is known as a “scheduling request” (SR), and typically comprises only a single bit to indicate that data is in the buffer. It is also known for an SR to comprise a small plurality of bits, which gives greater functionality. In response to receiving an SR, a primary station may either transmit a grant allocating a suitable amount of transmission resource for the secondary station then to send a BSR, or transmit a grant allocating a larger amount of transmission resource which would enable the secondary station to transmit some data in addition to the BSR; however, in the latter case there is typically no means for the primary station to determine a suitable size of allocation to make, unless the SR comprises more than a single bit.
As a consequence, the known implementations for informing the primary station of the status of the secondary station's buffers do not make it possible to have an adequate amount of information. Indeed, in cases of low activity of the secondary station, the BSR transmitted regularly are more than sufficient, and do not use the resources in an efficient manner. On the contrary, in case of high activity of the secondary station, the BSRs may not give sufficient information of the real state of the secondary station's buffers.